1. Field of the Invention
The present invention relates to a battery pack including battery stacks each formed of unit batteries stacked on one another, the battery packs being arranged in a battery pack case.
2. Description of Related Art
It is known to mount a battery pack case on a hybrid vehicle which has, as a vehicle driving source, a combination of an internal combustion engine and a battery-driven motor, or an electric vehicle, the battery pack case housing secondary battery cells for powering a vehicle-driving motor.
The secondary battery cells generate heat due to Joule heat due to a current and chemical reaction when charged or discharged. This heat generation occurs mainly at the vicinity of the electrode terminals of the battery cells. Accordingly, the above battery pack is configured to use its bus bars or the like directly connected to the electrode terminals as heat transmission paths for dissipating the heat to coolant such as air.
For example, the battery pack described in the Japanese Patent Application Laid-open No. 2007-329047 has a structure in which a spacing is provided between each adjacent two of the battery cells, and the cooling air blown from a fan flows through the spacings so that the cooling air hits the entire of the heat dissipation surface of each of the battery cells as much as possible. Incidentally, this battery pack is configured such that the cooling air does not leak in directions other than the cooling direction to provide a high cooling performance. Further, this battery pack includes bus bars, cables and an exhaust duct at places other than the passage of the cooling air.
However, the battery pack described in the above patent document has a problem in that the amount of heat transmission from the battery cells to the inner surface of the battery pack case has to be increased by increasing the circulation amount of the cooling air with the reduction of the size of the battery pack case.
On the other hand, to reduce the size of the battery pack case, the bus bars, cables and exhaust duct have to be disposed in the vicinity of the electrode terminals. In the so-called vertical flow type of a battery pack in which cooling air is blown from the upward side to the downward side with respect to battery cells, electrode terminals and bus bars (or an exhaust duct or an electronic component in some cases) which are disposed on the upward side become an obstacle to the flow of cooling air, causing the cooling performance to be degraded.